Accelerations in motor vehicles, particularly in their airbag control units, are measured using micromechanical inertial sensors. In so doing, the principle of the differential capacitor is used, in which a movable seismic mass, together with fixed reference electrodes, forms two capacitances. If an acceleration acts on the mass, it is deflected and the capacitances change. The difference in the capacitances is converted by an electronic circuit, a so-called capacitance/voltage converter (C/U converter), into a voltage signal essentially proportional to the acceleration. The movable structures and the capacitor-like arrangements, which form the capacitances, are usually implemented as microelectromechanical structures (MEMS).
The precise centering of the seismic mass between the fixed electrodes represents a problem when manufacturing the inertial sensors in the construction as MEMS. As a rule, the actual position of the seismic mass deviates from the desired central position due to process uncertainties. This displacement leads to a capacitive signal which is not owing to a deflection of the mass as a result of an influencing acceleration, and which is usually compensated for by an electrical circuit. To that end, the deviation is electrically measured when switching on the sensor and is continually subtracted from the output signal during operation.
During normal operation, the seismic mass of the inertial sensor is induced by accelerations to movements having a certain amplitude and in part high frequency.
In unusual operating states, e.g. when the sensor is subjected to strong impacts in the provided deflection direction of the seismic mass, the amplitude can be very much greater. To prevent contact of the electrodes and therefore an electrical collapse, mechanical stops are provided which limit the deflection of the seismic mass. Because of the circumstance that the position of rest or vibrational center position of the seismic mass deviates from the geometric center position between the stops, an asymmetrical limiting of the sensor signal occurs, resulting in a faulty signal at the sensor output.